Young person's railcard valid more than the Network card, so you can use it until you are too old for that - but be prepared that Network railcard can't be used til 10, even though off-peak starts after 9 ish (depending on direction etc).
Fast trains to/from Kings Cross are about 45 min but generally more expensive during peak times. Slower trains to/from Liverpool St have various pricing options with singles, advanced, super-advanced and all that, even during the peak times, so if you book early you can get some better deals but it takes ages.
I think there are (or are plans) to have some sort of Carnet tickets where you can use them a certain number of times within a time period, rather than a general season ticket.
Not all the trains go through to Cambridge North, so it depends where you are working, if the central station is close enough or if you want the North one (Kings Cross trains tend to go on to there). Soon to be a South station too if you work at biomedical campus, which will take time off the journey from London and possibly from the Cambridge end as well. If you can cycle from the Cambridge end to work, you will likely save time over a bus, unless you work within walking distance.